Method for desealing integral fuel tanks



States atent Fine 3,591,348 METHOD FOR DESEALING INTEGRAL FUEL TANKS Hans Frischknecht, Downey, Calif., assignor, by mesne assignments, to Chemetron Corporation, Chicago, Ill., a corporation of Delaware No Drawing. Filed Aug. 1, 1966, Ser. No. 569,061

Int. Cl. B0811 9/08 US. Cl. 134-22 7 Claims ABSTRACT OF THE DISCLOSURE Rapid removal of dried and cracked synthetic rubber sealing compounds that line integral fuel tanks by spraying a water-borne chemical formulation of organic solvents, emulsifiers and activating agents thereon for approximately the same period of time as would be required to remove such a sealing liner by means of said chemical formulation alone, and water-flushing away the particles thus dissolved and removed from the liner.

This invention relates to a method for removing the sealant that lines the interiors of integral fuel tanks, particularly of aircraft.

The compounds that line integral tanks leak-proof them so they may efficiently carry highly volatile fuel. These sealant compounds vary in their composition, examples thereof being lead-peroxide-cured Thiokol, dichromatecured Thiokol, and Buna-N synthetic rubber. In time, such sealants become dried, cracked, and develop leaks that render them ineflicient and unsafe for holding highly volatile fuels. Their removal and replacement by fresh lining sealant, therefore, becomes necessary.

The present invention relates, primarily, to the recirculating spray method of descaling integral fuel tanks of aircraft, wherein a desealant solution is drawn from a reservoir by a pump, is sprayed onto the wall of a fuel tank, and is returned to the reservoir, passing through a screening device or filter to remove solid particles of removed sealant before it passes through the pump again on its way back to the fuel tank wall. The reservoir holds about three times as much fluid as the pump will deliver in one minute. This rapid movement of fluid contributes to the success of this invention.

Any time, through the years, that Water became intermixed with the descaling solvent, the mixture was discarded, in the universal belief that such a solvent-water fluid would not deseal integral fuel tanks. Several primary factors contributed to the disinterest of the industry in use of desealant mixed with water. The first was the reluctance to chance spoiling, for further use, hundreds and even thousands of gallons of expensive descaling solution in the firm belief, as above stated, that the same would represent a substantial monetary loss, because water would ruin the desealant, causing it to be so ineffective that the tanks would be only partly desealed at best, and, perhaps, remain in the same poorly sealed condition as before such desealing was attempted. It was expected that, at best, a water-mixed desealant, if it was effective on synthetic sealants at all, would be so only over such long periods of time that the costs would mount, and no one wanted to risk incurring the excessive costs resulting from a long increase in lay-up time of the airplanes.

Desealing the fuel tanks of an airplane entails two major expendable items: time to perform the job, and cost of materials to perform the job. The present invention reduces the cost of desealant materials by at least at no apparent increase in the time, either in man hours or in tie-up time of the aircraft. This is accomplished by mixing the desealant material with appreciable amounts of water. The savings that can be realized not only include the direct reduction in amount of desealant used on a job, but reduce inventories, freight, and other handling costs. The admixture of water with the desealant material on the site, just prior to use, permits the work to be accomplished with no more than half the desealant material that would be required to accomplish the same work if the desealant material were to be used full strength, as is the current practice.

Accordingly, an object of the present invention is to provide a desealing method for integral fuel tanks that obviates the above costly procedures of prior descaling methods and enables eflficient desealing employing a desealant solution that includes water in appreciable amounts.

Another object of the invention is to provide a method, as characterized, in which the heretofore large amount of desealant solvents is materially reduced, thereby not only lowering the cost of the solvents used in a descaling operation, but also eliminating the expense of shipping such unused solvents from the supplier thereof to the locality at which the descaling operation takes place.

The invention also has for its object to provide a novel, economical and convenient method or process of superior utility.

The invention also comprises novel combinations of method steps, which will appear more fully in the course of the following description of the present method or process.

The present invention has basis on the discovery, during laboratory tests, that fuel tank sealants could be removed at a practical rate with a formulation that comprises a liquid chemical composed entirely of organic solvents, emulsifiers, and activating agents, intermixed with an appreciable amount of water.

In laboratory tests, panels were covered with two types of fuel tank sealant. One type was lead-peroxide-cured Thiokol, and the other was dichromate-cured Thiokol. After these coatings were allowed to become throughly cured, a solution of organic solvents, emulsifiers, and activating agents was continuously sprayed on each panel, as follows:

Percent by weight In approximately one hour, both types of panel had been stripped of their sealant coatings.

The aromatic thiols, more specifically, were composed of 85% xylene thiol and 15% aliphatic petroleum solvent. The sodium sulfonate which is an organic sodium sulfonate having a molecular weight of 325 to 425 and is of the type disclosed in column 3, lines 11-17, in United States Patent No. 2,852,471, granted to Don C. Atkins, Jr. et al. on Sept. 16, 1968, was about to active, with the remainder being petroleum oil. Of the 5% given above, the organic sodium sulfonate would comprise 3% and the oil 2%.

The alcohol isopropanol is not an important ingredient in the above desealant, insofar as the efficiency of the desealant is concerned. Its primary purpose is to cut down on the proportion of the methylene chloride which otherwise may come to 79% to 80% of the formulation, the proportion of thiols and sodium sulfonate remaining as given. Therefore, for the purposes of this invention the chemical formulation commonly used for desealing tank walls sealed with synthetic rubber may be described as a desealant composed of organic solvent (methylene chloride), emulsifiers (an organic sodium sulfonate), and activating agents (aromatic thiols).

The above formulation is old and is given merely as an example of a desealing solution used in the trade for desealing integral fuel tanks, and which the trade refused to believe could be mixed with water and still be effective to deseal such tanks.

As mentioned, the above exemplary, but old, formulation desealed the mentioned panels so the dissolved sealant could be flushed away with water, as had been the practice in the field in the past.

Then, two test panels were similarly covered, as above, one with lead-peroxide-cured Thiokol rubber, and the other with dichromate-cured Thiokol. After thorough curing of these coatings, a mixture of the solution of organic solvents, emulsifiers, and activating agents, as given above, and an equal amount of water, was continuously sprayed, as before, on each said coated panel. This 50% water-50% solvent mixture stripped the sealants therefrom in the same approximate time of one hour.

Tests carried out with a smaller proportion of water in the mixture still required an hour to remove these sealants, but when the proportion of water was increased, desealing took commensurately longer after about 60 parts of water to 40 parts of desealant were used, and ultimately the operation became inefficient for practical desealing.

These tests made it clear that just as efiicient desealing of Thiokol-lined tanks may be effected with a mixture of 50% to 60% of water and 50% to 40% of solvent-emulsifier-activating-agent base solution, as may be achieved with such a base material alone.

Additional tests were carried out to determine where the desealing efficiency varied if, in fact, it did vary, and at what level of water-sealant ratio desealing efficiency began to lower. As before, the test panels were covered with lead-peroxide-cured Thiokol and with dichromatecured Thiokol and thoroughly cured. The above-defined desealant was mixed with 3% water, then 5% water, and then water and applied to said panels, as before, the same one hour time was required to strip the sealants from the panel as was required with the 50% water, 50% solvent or desealant mixture, as above. Thus, the efficiency of the desealant liquid from 0% to 50% of water remained at the maximum level of 100%. However, the cost of the basic desealant decreased from maximum at 0% water to 50%. The above is given without consideration of the water cost which, only in rare instances, may have a slight effect on the cost of the water-solvent liquid.

Continuing the tests, as above, with the water percentage increased in small increments, the above maximum efiiciency was maintained until about 60% of water to 40% of desealant was used. Then, the stripping efiiciency fell off proportionally as the water percentage was increased. Therefore, 50%60% water represents the optimum etficiency range of the water-desealant liquid.

It is common practice to seal an integral tank by first covering the joints and other raised and projecting places and areas, such as bolt and rivet heads, ribs, and the like, with Thiokol, as in the examples hereinabove given, and then coating these and all the other surfaces of the tank with Buna-N synthetic rubber. Tanks so sealed have been stripped with exemplary desealants of the formulation set out above. A panel thus coated was subjected to a spray of pure or undiluted desealant for about five minutes, at which time the Buna-N cover coat was removed and a portion of the Thiokol sealant was dissolved. A water rinse was then applied and the panel then soaked in water for about fifteen minutes to effect thorough water saturation of the residual sealant. Pure desealant was then applied and effected a complete stripping of the sealant in the same time that was required before.

By this test, it was shown that the water and desealant could be applied separately and sequentially, rather than by mixing them to form a desealing liquid, as above described. In other words, alternately applying desealant and rinse Water to a sealed surface, rather than applying them together, did not affect the efficiency and stripping time of the desealing operation.

The present method, based on the first example, has been caried out several times in the field on integral fuel tanks, to successfully strip them of various synthetic rubber linings. In each case, at least half of the fluid used comprised water, and the remainder a base solution of solvent-emulsifier.

While small test panels could be stripped in one hour in laboratory tests, the stripping time of actual fuel tanks, with walls of much larger area, took from two to six hours of recirculation of the water-solvent mix. It takes a comparable length of time to effect removal of sealant using full-strength desealant.

As explained, the base solutions, per se, form no part of this invention, since they are varied as desired, depending upon the sealant to be dissolved and stripped. It is only necessary, in carrying out the present method, to use the same base solution for a particular desealing operation as has been used in the past for the particular sealant, with the important and novel exception that to 60% of water is substituted for a like amount of solution, thereby having a water-solution mixture that may range downwardly to 25% water from a maximum of 60% water for formulation as given herein.

While the foregoing discloses what are now contemplated to be the best modes of carrying out the invention, the same are, of course, subject to modification without departing from the spirit and scope of the invention. Therefore, it is not desired to restrict the invention to the particular method steps or combination or sequence of method steps described, but to cover all equivalent steps or methods that may fall Within the scope of the appended claims.

Having thus described the invention, what is claimed and desired to be secured by Letters Patent is:

1. In the method of desealing the synthetic rubber, sealant-covered walls of fuel tanks in which a desealant composition is recirculated between the interior of the fuel tank and a reservoir outside of the fuel tank and is sprayed over the interior Walls of the fuel tank, the improvement which comprises recirculating a desealant composition consisting essentially of an admixture of at least one organic solvent for the synthetic rubber sealant covering the walls of said fuel tank, an emulsifier, an activating agent and from 25 to 60 percent water based on the weight of the mixture, repeatedly recirculating said desealant composition until the walls of said fuel tank are desealed, and thereafter, removing said desealant composition and dissolved and loosened sealant in said desealant composition from said fuel tank.

2. The method of claim 1 wherein said emulsifying agent is an organic sodium sulfonate having a molecular weight ranges of 325 to 425.

3. The method according to claim 2 in which the sodium sulfonate has a molecular weight between about 325 and 425 and is to active, the remainder being petroleum oil.

4. The method according to claim 3 in which the oil comprises about one-third of the 5% sodium sulphonate.

5. The method of claim 1 wherein the activating agent is an aromatic thiol.

6. The method according to claim 5 in which the aromatic thiol comprises xylene thiol and 15% aliphatic petroleum solvent.

7. The method of claim 1 wherein said desea1ant composition contains 50 percent water based on the weight of the mixture.

References Cited UNITED STATES PATENTS 9/1953 Whitcomb 134-22 XR 9/1958 Atkins 13422 XR 6 OTHER REFERENCES I. Mellan: Source Book of Industrial Solvents, Reinhold, 1957, pp. 119-120.

MORRIS O. WOLK, Primary Examiner D. G. MILLMAN, Assistant Examiner US. Cl. X.R. 134-38; 252-364 

